Linear Metal−Metal-Bonded Tetranuclear M−Mo−Mo−M Complexes (M = Ir and Rh):  Oxidative Metal−Metal Bond Formation in a Tetrametallic System and 1,4-Addition Reaction of Alkyl Halides

Reaction of Mo₂(pyphos)₄ (1) with [MCl(CO)₂]₂ (M = Ir and Rh) afforded linear tetranuclear complexes of a formula Mo₂M₂(CO)₂(Cl)₂(pyphos)₄ (2, M = Ir; 3, M = Rh). X-ray diffraction studies confirmed that two “MCl(CO)” fragments are introduced into both axial sites of the Mo₂ core in 1 and coordinated by two PPh₂ groups in a trans fashion, thereby forming a square-planar geometry around each M(I) metal. Treatment of 2 and 3 with an excess amount of ^tBuNC and XylNC induced dissociation of the carbonyl and chloride ligands to yield the corresponding dicationic complexes [Mo₂M₂(pyphos)₄(^tBuNC)₄](Cl)₂ (5a, M = Ir; 6a, M = Rh) and [Mo₂M₂(pyphos)₄(XylNC)₄](Cl)₂ (7, M = Ir; 8, M = Rh). Their molecular structures were characterized by spectroscopic data as well as X-ray diffraction studies of BPh₄ derivatives [Mo₂M₂(pyphos)₄(^tBuNC)₄](BPh₄)₂ (5b, M = Ir; 6c, M = Rh), which confirmed that there is no direct σ-bonding interaction between the M(I) atom and the Mo₂ core. The M(I) atom in 5 and 6 can be oxidized by either 2 equiv of [Cp₂Fe][PF₆] or an equimolar amount of I₂ to afford Mo(II)₂M(II)₂ complexes, [Mo₂M₂(X)₂(^tBuNC)₄(pyphos)₄]²⁺ in which two Mo−M(II) single bonds are formed and the bond order of the Mo−Mo moiety has been decreased to three. The Ir(I) complex 5a reacted not only with methyl iodide but also with dichloromethane to afford the 1,4-oxidative addition products [Mo₂Ir₂(CH₃)(I)(^tBuNC)₄(pyphos)₄](Cl)₂ (13) and [Mo₂Ir₂(CH₂Cl)(Cl)(^tBuNC)₄(pyphos)₄](Cl)₂ (15), respectively, although the corresponding reactions using the Rh(I) analogue 6 did not proceed. Kinetic analysis of the reaction with CH₃I suggested that the 1,4-oxidative addition to the Ir(I) complex occurs in an S_N2 reaction mechanism.